My mountain bike is newly rebuilt (by me) with all brand-new drivetrain and wheels, and it is working well, but it does just ONE really annoying and possibly dangerous thing. Sometimes, maybe once or twice per day, when I'm coasting fast, the cassette will slowly start turning "forward" with the wheel, which makes the top stretch of chain go slack and hang down on the chainstay, and maybe skip across a gear as the slack is yanked back. When I hear it starting to do this, I pedal forward a couple turns and it returns to normal. Is this likely to be a lubrication issue, even though the parts are brand-new, or is something possibly too tight somewhere?

dude. sounds like the freewheel is to tight. there are two bolts on either side of the axle. if u tighten them to tight, the cogs wil try to turn with the wheel. take wheel off and spin it and hold in ur hands. if there is any resistance, thoes nuts are too tight. really ull ruin ur new gear if ur not careful so read up on all u can find.

Contrary to other opinions here, it sounds like there is too much grease in the freehub assembly, especially if it is already "exuding plenty of grease at the seams" as you say. Heavy grease can "grab" at the internals and transmit force much like hydraulic fluid in a car's automatic transmission.

NJWheelBuilder's suggestion to soak it in solvent might help, especially if you follow that up with a light oil.

Heh, you should have seen all the grease that came out of one of the new pedals. The guy at the factory went nuts with the grease ***. I'm going to monitor it for now, and try to figure out what's going on without disassembly.

The cassette is locked on with a disk nut that tightens against a knurled surface on the cassette - should I worry about wearing down this knurled surface by taking the cassette off too often?

When I grab the nut on the end of the rear axle and turn the axle, every few turns it becomes hard to turn for a just moment, like it's going to bind up. I think this might be related to the problem of the cassette turning clockwise a bit causing top chain slack when coasting. I tried to follow my maintenance book's instruction on "flushing" the freehub with lube, but it didn't seem like enough lube was going into the mechanism to "flush" in any real sense. I also didn't see how any significant amount of dirt could have got into the hub. Fortunately it's a rare problem, possibly because all the off-road bouncing helps to retension the chain quickly.

We had a nice ride around hunting/fire road at the 9000 foot level of Mauna Kea today. Saw feral pigs, roaming dogs, lots of birds, and a patch of little silverswords. No people.